1. Field of the Invention
This inventions relates to compensation circuits for demodulators, and particularly to circuits for intermediate frequency (IF) quadrature demodulators.
2. Discussion of the Background Art
Mobile phones are broadly classified as cellular or PCS (Personal Communication System). A cellular phone system operates at a carrier frequency of 900 MHz, while PCS operates at double this frequency, or 1.9 GHz. Analog modulation is Frequency Modulation (FM), while digital modulation includes CDMA (Code Division Multiple Access), TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access).
Quadrature modulation/demodulation refers to modulation/demodulation of orthogonal carrier signals. Modulators and demodulators are built on a monolithic semiconductor chip. A block diagram of a typical prior art quadrature demodulator is shown in FIG. 1. The demodulator includes two mixers 101 and 102, both of which comprise a signal input, an input from local oscillator 108 and a low frequency output. A modulated signal is fed to mixer 101, and LO signal from local oscillator 105 is also fed to mixer 101. The modulated signal is also fed to mixer 102, while local oscillator signal LO is fed to mixer 102 through a phase shifter 103 that produces a 90.degree. phase shift with respect to the local oscillator signal LO fed to mixer 101.
The local oscillator 108 has a tendency to leak its signal LO into other parts of the demodulator system. It is often difficult to balance mixers 101 and 102 such that the level of an incoming carrier signal and the local oscillator 108 leak into a modulated high frequency signal are small enough with respect to the incoming signal v.sub.i (t). A local oscillator signal leak creates a significant distortion in the voice signal.
U.S. Pat. No. 5,012,208 to Makinen discloses a quadrature modulator having compensation for local oscillator leak. Makinen utilizes a circuit adding different compensation voltages to two modulator signals independently of one another on the basis of the correlation between amplitude variation of the output signal and the corresponding modulation signal. However, this patent is concerned with a modulator rather than a demodulator, and does not provide a frequency offset in the local oscillator.
Other prior art devices perform dc offset compensation by shutting off the RF input signal during a guard space between TDMA slots. Still, this technique is not effective when the signal is present due to the inherent dc offset from the modulated signal itself. Thus, there is a need to utilize dc offset compensation in the presence of the received signal.
More generally, other parts of the signal contribute to producing dc components. These other dc components interfere with removal of the unwanted dc component from local oscillator leakage. It is therefore necessary to compensate for these other dc components prior to removing the unwanted dc components from local oscillator leakage.